Torque Setting

I'd simply set up a spare bolt, torque it past the elastic limit, see what that was, and go for 50% of that.

Depends on what the materials its holding together are though: the two reasaons to torque up massively are to prevent loosening under vibration, but anything that goes beyond te elastic limit of the weakest material being bolted, is a waste, and to prevent slippage laterally.

Vibration can in any case be catered for with threadlock.

If the structure is specced for 71NM, then that would work fine and be as per spec. In this case you could apply more, but doing so could damage something that youre bolting and cause failure. If the design says 71NM, 71 it is.

NT

Ok being from an engineering background, I would have thought that the torque setting is not neccessarily the grade of galvanising but more for the actual size of the bolt/screw in a given situation. But then again I may be wrong.

I don't think the 'grade' refers to the galvanising, but rather to the grade of steel of which the bolts are made. Higher tensile steel can take higher loads - size for size - but, as others have said, too high a torque may damage the materials being clamped - so the value appropriate for the originally specified bolt is the one to use.

That would be my interpretation. The OP needs to clarify.

But is it? Would a higher tensile strength bolt exhibit the same clamping force as a lower tensile strenght one for the same applied torque? I should think (but don't know) that provided *both* bolts are not getting into their elastic area of tension then the force will be the same but some bolts are designed to be used in their elastic region...

Whichever, thats a fair amount of torque - what are you building if you don't mind me asking.

Use the design torque, it may not be the bolts that are the weakest link. What does the designer/supplier say?

Of course. Its torque times atan pitch angle on the threads, all other things being equal, like the elastic limit of the steel not being reached..

I think you are wrong even if it doesn't make sense.

No bolt should be used out of its elastic region, if you ever want to reuse it.

Bolts should be used in their ELASTIC region, then torque and clamping force are always constant and related directly by thread pitch.

And the coefficient of friction in the threads, and under the head of the bolt.

Normally not that great an effect. you would need a coefficient of friction of 1 to achieve a 40% greater torque than the above shows..typical steel on steel is a lot less..about 0.1 from memory.

I think you mean plastic rather than elastic. Assuming the same thread pitch, the same torque will give the same clamping force.

A stress engineer and a designer once told me that making something stronger in any construction can make other parts of it weaker. I'd go for 71NM.

Dave

I may have the defintions arse about face... There are certainly some bolts that you do not reuse because the torquing up deliberatly takes them past the elastic limit. that is they become permenantly stretched. Head bolts of some Land Rover engines spring to mind.

Alas not quite so simple if you are dealing with lubricated or high temperature fasteners (or both). Also the torque contribution from under the head is exactly linear with friction coefficient. Given that the effective radius with a hex head bolt (as opposed to a socket head screw) is much larger than the thread radius, that term can be quite significant.

Bolts don't have torque settings. Fastenings (the overall construction) has a torque setting, bolts just have a _maximum_ torque. If you replace a monkeymetal bolt with an unobtainium bolt, then all other things being equal, the torque setting should remain the same.

Underlying all this, fastenings don't have a torque setting either. They have a compressive force you want to achieve, which equates to applying a particular torque to a bolt with a particular surface and helix angle. Something as simple as clean / dirty / oiled / loctited / zinc plated surfaces can change this relationship.

So in general, for bolts that are otherwise consistent, stick with the design torque for the fastening even if the bolt material changes. If you _downgrade_ materials, the bolts might then shear before they get to design torque for some uses. If you have a critical application, you'll also need to check the specs for the surface condition before assembly.

If you have "torque to yield" bolts, then use _exactly_ the ones necessary. Equally for angular torque settings ("hand tight then

I don't use many M16s. When I do, and there's an external "specification" involved, it tends to be for structures that require liability insurance etc. In these cases you need good practice _and_ you need a paper trail to cover you in case it runs amok and eats children (or round here, if the children run amok, torch it and then try to sue you).

So if there's any question, clear it with the supplier beforehand. This isn't because they'll give the best answer, it's because they're the answer you can hide behind in court.

Sounds reasonable.

If you're rivetting a structure, you use lots of crude fasteners and spread the necessary overall load amongst them. Rivets are not usually a particularly high performance material.

If you're bolting a structure, especially these days, the tendency is to use fewer fasteners. This gives better assembly speed, but it puts more demands on each fastener. If you're placing the load of 20 rivets through one bolt instead, it may well _need_ to be high-tensile.

So if the majority of your bolt market is going to need high-tensile anyway, why stock the cheap & cheerfuls? You can always replace a low- tensile with a high-tensile, and (as discussed above) it's usually a simple drop-in replacement at the old settings.

That is deliberately done top prevent warping of the alloy they are clamping.

Not the issue here.

Mind you, many people never re-use bolts..certainly the competitive automotive people don't.